1. Field of the Invention
The present invention relates generally to high-current switches and more particularly to crossed-field plasma switches and associated switching methods.
2. Description of the Related Art
High-current switches are described in Opening Switches, Plenum Publishing Corp., New York, 1987, edited by A. Guenther, et al. In particular, a chapter, written by Robert W. Schumacher and Robin J. Harvey and entitled "Low-Pressure Plasma Opening Switches", describes two examples of high-current plasma switches.
The first is the crossed-field tube (XFT). XFTs are low-pressure plasma-switch devices which change the magnitude of a magnetic field to control conduction. Their structure generally includes a cylindrical anode and a surrounding cylindrical cathode. An electromagnet is typically wound in intimate contact with the cathode outer surface and the interelectrode gap is filled with a low-pressure gas.
In operation, voltage is applied across the electrodes to establish a radial electric field E. Current is applied to the electromagnet to generate an axial magnetic field B that is nearly uniform in the switch volume. In this crossed-field geometry, the electrons in the interelectrode gap move along a substantially circumferential path where they collide with gas atoms to produce secondary electrons and ions. The length of this path causes a sufficiently large number of collisions to ignite and maintain a plasma. Removing the magnetic field B causes the electrons to move along a radial path between the electrodes. Because this radial path length is too short to produce significant numbers of secondary electrons, the plasma then dissipates. The switch opening is delayed by the time it takes for the magnetic field to fall throughout the entire switch volume and for the residual plasma to flow to the containment walls. This delay is further increased because of local eddy currents in the walls.
A basic XFT embodiment was described in U.S. Pat. No. 3,678,289, which issued Jul. 18, 1972 to Michael A. Lutz, et al. and was assigned to Hughes Aircraft Company, the assignee of the present invention. In this XFT, a permanent magnet produces an axial magnetic field. A second, switchable electromagnet is arranged to generate a pulsed magnetic field that axially opposes ("bucks") the permanent field. The reduction in the magnetic field opens the switch.
XFT arcing problems were addressed in U.S. Pat. No. 3,749,978, which issued Jul. 31, 1973 to Hayden E. Gallagher and was assigned to Hughes Aircraft Company. This patent discloses that arcing problems can be reduced if the net magnetic field is maintained beneath the critical value for a sufficiently long time duration.
A specific bucking coil embodiment is disclosed in U.S. Pat. No. 3,873,871, which issued Mar. 25, 1975 to Gunter A. G. Hofmann and was assigned to Hughes Aircraft Company. This patent shows a short circuit coil associated with the bucking coil and the permanent field coil to reduce inductive coupling.
Another bucking coil embodiment is shown in U.S. Pat. No. 4,071,801, which issued Jan. 31, 1978 to Robin J. Harvey and was assigned to Hughes Aircraft Company. In this patent, a bucking coil is oriented orthogonally with both the electric field and the main magnetic field to reduce the energy required to develop the opposing field.
XFTs can carry and interrupt very large currents, e.g., &gt;1000 amperes. However, their interrupt time is rather long, e.g., 10 microseconds. Since the main field can be quite strong, e.g., &gt;100 gauss, a substantial bucking field pulse is required to bring the net field strength below the critical value for ionization. Coils to produce large pulsed fields have substantial inductance; consequently, current pulses through them have slow rise and fall times.
A second plasma switch example is the CROSSATRON Modulator Switch (CMS) (CROSSATRON is a trademark of Hughes Aircraft Company). The CMS uses a low-pressure, crossed-field discharge to generate a high-density plasma for conducting high currents with low forward drop across the switch.
The CMS typically has two grids, a source grid and a control grid. These grids are usually cylindrically shaped and coaxially arranged with the cathode and anode. They are positioned between the cathode and anode with the source grid adjacent the cathode. Magnets are positioned around the cathode to establish a magnetic field B that is preferably limited to the gap between the cathode and the source grid. The CMS controls the conduction of electrons to the anode by controlling the potential of the control grid. Therefore, the CMS controls current by the application of an electric field while the XFT controls current by the application of a magnetic field.
The basic structure of the CMS is described in U.S. Pat. No. 4,247,804, which issued Jan. 27, 1981 to Robin J. Harvey and was assigned to Hughes Aircraft Company.
Methods and structure directed to controlling a CMS were disclosed in U.S. Pat. No. 4,596,945, which issued Jun. 24, 1986 to Robert W. Schumacher, et al., and was assigned to Hughes Aircraft Company. This patent found that successful current interruption in a CMS depends upon the use of low gas pressure and upon the physics of the control grid-plasma interface.
An improved cold cathode structure was disclosed in U.S. Pat. No. 5,019,752, which issued May 28, 1991 to Robert W. Schumacher and was assigned to Hughes Aircraft Company. Generation of secondary electrons was enhanced by a cathode configured with a series of perturbations.
CMSs have been constructed that are capable of holding off voltages up to 100 kV and of interrupting currents up to 1000 Amperes. However, all CMSs can conduct more current than they can interrupt. Consequently, a fault condition in a CMS-based switching system may cause the CMS current to overwhelm the capability of its control grid to interrupt the current. At best, this requires restarting the system; at worst, the system or parts being processed by the system are damaged.
For example, an exemplary ion-implantation system is configured to deliver negative, high-voltage pulses to a part that is immersed in a plasma field. Ions in the plasma are accelerated toward and implanted into the part. However, the part's surface condition sometimes causes an arc which reduces the circuit load. The CMS cannot interrupt the resulting high current; as a consequence, stored energy in the system is dumped and the part is severely damaged. Because the system conduction path will conduct significant current within 10 microseconds, preventing parts damage would require the ability to interrupt fault currents in less than 5 microseconds.